In series with this is an adjustable rheostat CB. The maximum value r5 of this resistance is chosen equal to the maximum value of the resistance of the line and couple likely to occur in practice. A convenient value is 15 ohms. With the slide of the rheostat set for the maximum resistance, ri = rb the instrument is calibrated in terms of the potential drop across AH. Hence when the instrument is connected through the line of resistance L to the couple of resistance T
FIG. 5.—Harrison-Foote compensated indicator.
the rheostat CB must be adjusted until TI + L + T = r5. When this adjustment is made the scale reading gives correctly the electromotive force e of the couple, or the correct temperature if the scale is graduated in degrees.
This adjustment is made in the following manner. By depressing the key K, a portion of the galvanometer resistance r2 is short circuited and the rest of the resistance ra containing the moving coil, is shunted by a resistance ri. If e' represents the potential drop across DF when the key is open and e" represents the potential drop when the key is closed, we obtain:
' L.......+ T......+ <
°" = CL—
+ r2 + rs r4
If ri is so adjusted that these two potential-drops and hence the deflections of the indicator are the same, we have on equating (1) and (2):
L •+• T + TI = ---* = a constant. (3)
If now in the construction of the instrument, r^r^/r^ is made equal to rc, the above setting makes L + T -f ?'i = ^5, the adjustment required in order that the reading of the scale gives the true electromotive force of the couple.
The ease with which the proper setting can be obtained is greatly improved by making r3/r4 equal to from 5 to 10. Suppose r-^/r^ is made equal to 9. Then if r\ is improperly adjusted and the instrument reads in error by say 8e when the key K is open, on depressing the key the reading is changed by 9.5e. If now r\ is readjusted with the key depressed until the reading takes its initial value, the error with the key open is reduced to Se/10. The process for operating the instrument is accordingly as follows:
1. Read the instrument with the key open.
2. Close the key and adjust the rheostat n until the instrument reads approximately the same as in 1.
8. Repeat 1 and 2 if necessary. When r3/r4 = 9 it is rarely necessary to make a second adjustment. In position 1